U.S. patent number 7,135,678 [Application Number 10/887,800] was granted by the patent office on 2006-11-14 for charged particle guide.
This patent grant is currently assigned to Credence Systems Corporation. Invention is credited to Theodore R. Lundquist, Tzong Tsong Miau, Qinsong Steve Wang.
United States Patent |
7,135,678 |
Wang , et al. |
November 14, 2006 |
Charged particle guide
Abstract
A charged particle guide adapted to be coupled with a charged
particle detector, such as a secondary electron detector. The
charged particle guide, in one example, comprising two wires
extending from the charged particle detector toward a source of
charged particles, such as secondary electrons emitted from an IC
upon application of a focused ion beam. Upon application of a bias
voltage, the charged particle guide introduces a collecting
electric field that attracts charged particles and directs the
charged particles to the charged particles detector.
Inventors: |
Wang; Qinsong Steve (Saratoga,
CA), Miau; Tzong Tsong (Sunnyvale, CA), Lundquist;
Theodore R. (Milpitas, CA) |
Assignee: |
Credence Systems Corporation
(Milpitas, CA)
|
Family
ID: |
35540333 |
Appl.
No.: |
10/887,800 |
Filed: |
July 9, 2004 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20060006329 A1 |
Jan 12, 2006 |
|
Current U.S.
Class: |
250/310; 250/311;
250/397; 250/306 |
Current CPC
Class: |
H01J
37/244 (20130101); H01J 37/3005 (20130101); H01J
2237/2448 (20130101) |
Current International
Class: |
H01J
37/244 (20060101); H01J 37/28 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
"Yttrium Silicate--Cerium Doped P47" Applied Scintillation
Technologies, located at www/appscintech.com, Nov. 30, 2000. cited
by other.
|
Primary Examiner: Wells; Nikita
Attorney, Agent or Firm: Dorsey & Whitney LLP
Claims
The invention claimed is:
1. A charged particle detector assembly comprising: a charged
particle detector; a first elongate member coupled with the charged
particle detector; a second elongate member coupled with the
charged particle detector; the first and the second elongate
members extending away from the charged particle detector; and at
least one electrical connection point arranged to supply at least
one bias voltage to the first and the second elongate members; and
wherein the first and the second elongate members each comprise a
wire.
2. The charged particle detector assembly of claim 1 wherein the
charged particle detector comprises a secondary electron
detector.
3. The charged particle detector assembly of claim 2 wherein the
secondary electron detector comprises a scintillator.
4. The charged particle detector assembly of claim 3 wherein the
secondary electron detector comprises a ring arranged
circumferentially about the scintillator, and wherein the first
elongate member and the second elongate member are electrically
coupled with the ring.
5. The charged particle detector assembly of claim 4 wherein the
scintillator defines a disk shape defining an axis.
6. The charged particle detector assembly of claim 5 wherein the
first elongate member and the second elongate member are coupled
with the ring and each define a first section extending from the
secondary electron detector generally parallel with the axis of the
scintillator.
7. The charged particle detector assembly of claim 6 wherein the
first and the second elongate member each define a second section
electrically coupled with the first section, the second section
extending toward the axis of the scintillator.
8. The charged particle detector assembly of claim 7 wherein the
first and second elongate members each define a third section
electrically coupled with the second section, the third section
extending toward the axis of the scintillator at less an angle than
the second section.
9. The charged particle detector assembly of claim 8 wherein the
first and second elongate members each define a fourth section
electrically coupled with the third section, the fourth sections
extending substantially parallel to each other and further
extending at an angle with respect to the axis of the
scintillator.
10. The charged particle detector assembly of claim 1 wherein the
wire is of a material selected from the group comprising stainless
steel, Ni, Cr, Pd, and Pt.
11. The charged particle detector assembly of claim 1 further
comprising: at least one additional elongate member coupled with
the charged particle detector; the first, the second, and the at
least one additional elongate member extending from the charged
particle detector; and the at least one electrical connection point
arranged to supply the at least one bias voltage to the first, the
second, and the at least one additional elongate member.
12. The charged particle detector assembly of claim 1 wherein the
first elongate member and the second elongate member are
electrically insulated from the charged particle detector.
13. The charged particle detector assembly of claim 12 further
comprising a first electrical connection adapted to supply a first
bias voltage to the charged particle detector and a second
electrical connection adapted to supply a second bias voltage to
the first and the second elongate members.
14. The charged particle detector assembly of claim 13 wherein the
first bias voltage is different than the second bias voltage.
15. The charged particle detector of claim 13 where the second
voltage is less than the first voltage.
16. The charged particle detector of claim 15 wherein the first and
second voltages are positive to create a first positive collecting
electrical field and a second positive collecting field to attract
secondary electrons emitted from a sample.
17. The charged particle detector assembly of claim 1 wherein the
first elongate member is electrically isolated from the second
elongate member and wherein the at least one electrical connection
point comprises a first electrical connection to provide a first
bias voltage to the first elongate member and a second electrical
connection to provide a second bias voltage to the second elongate
member.
18. The charged particle detector assembly of claim 1 wherein the
charged particle detector assembly is adapted for use with a
charged particle tool having a platform for supporting a sample and
wherein the charged particle detector is arranged proximate to the
platform.
19. The charged particle detector of claim 18 wherein the first and
second elongate members are arranged to introduce a collecting
electric field proximate the sample supported on the platform.
20. The charged particle detector of claim 18 wherein the first and
second elongate members are arranged proximate the platform for
collection of secondary charged particles.
21. The charged particle detector of claim 1 wherein the first
elongate member and second elongate member are conductive and
whereby the bias voltage generates a collection electrical field
adapted to attract charged particles to the charged particle
detector.
22. A charged particle detector assembly comprising: means for
detecting charged particles; a first means for attracting charged
particles; a second means for attracting charged particles; a means
for electrically connecting the first means for attracting charged
particles with the second means for attracting charged particles;
and wherein the first means of attracting charged particles
includes a first wire extending from the means for detecting
charged particles and the second means for attracting charged
particles includes a second wire extending away from the means for
detecting charged particles.
Description
FIELD OF THE INVENTION
The present invention involves a charged particle guide, and more
particularly involves a charged particle guide for enhancing
collection of secondary electrons in a focused ion beam tool.
BACKGROUND
Charged particle beam systems, such as focused ion beam ("FIB")
systems, have found many applications in various areas of science
and industry. For example, in the semiconductor industry, FIB
systems are used for integrated circuit ("IC") probe point
creation, circuit editing, failure analysis, and numerous other
applications. A FIB tool typically includes a particle beam
production column designed to focus an ion beam onto the IC at the
place intended for the desired intervention. Such a column
typically comprises a source of ions, such as Ga+, produced from
liquid metal. The Ga+ is used to form the ion beam, which is
focused on the IC by a focusing device comprising a certain number
of electrodes operating at determined potentials so as to form an
electrostatic lens system. Other types of charged particle beam
systems deploy other arrangements to produce various charged
particle beams.
Successful use of a FIB tool depends, in varying degrees, on
obtaining high resolution images of the IC or other sample. The
images allow the user to view the IC during use of the FIB tool.
Various phenomena, such as secondary electrons, ions, neutrons and
photons, are available for monitoring FIB editing and generating
images. Secondary electrons, in particular, are emitted as a result
of the ion beam incident upon the IC. A common type of the
secondary electron detector ("SED") in FIB systems involves an
Everhart-Thornley type design using scintillator. A scintillator
typically includes a thin glass disk coated with a phosphor that
converts energy from secondary electrons into light photons. The
scintillator collects some of the secondary electrons emitted from
the IC and generates photons responsive to the secondary electrons.
In the photomultiplier tube, each photon generates multiple
electrons, which are then used to generate an image.
Different material characteristics provide different numbers of
secondary electron emissions. For example, with regard to an IC, a
dielectric emits substantially less electrons than a metal.
Typically, the greater the number of electrons, the brighter the
image. Lack of electrons provides a dark image. By rastering the
ion beam in a grid-like pattern, the contrast differences are used
to generate an image of the target portion of the IC. To generate a
clear and accurate image, the secondary electron collection
efficiency is an important aspect of any FIB tool. Oftentimes, a
large portion of the secondary electrons are emitted away from the
SED, making collection difficult.
A practice referred to as "circuit editing" is one example of a use
for a FIB tool. Circuit editing involves employing an ion beam to
remove and deposit material in an IC with precision. Through
removal and deposit of material, electrical connections may be
severed or added, which allows designers to implement and test
design modifications without repeating the wafer fabrication
process. Due to the small scale of the circuit editing process, its
success depends strongly on FIB image quality, which, as discussed
above, is directly linked to the number of secondary electrons
detected by the secondary electron detector.
Circuit editing success also depends on a process referred to as
"endpointing." Endpointing involves determining when to stop the
FIB milling operation. It is the objective of the operator to stop
the milling process at an interface at which the secondary electron
signal changes. In one example, endpointing involves detecting the
secondary electron signal as the ion beam drills down into the IC.
The emission volume is dependent on the material the beam is
milling. As mentioned above, metal emits a greater number of
secondary electrons than a dielectric. Thus, if the electron
emission characteristics are detectable, then boundaries between
dielectrics and metals are detectable. The current used to generate
an ion beam determines the power of the beam and the size of the
hole generated by a beam. As vertical interconnects in an IC get
laterally smaller, the ion beam etching current must be decreased.
Besides reducing the hole size, the secondary electron signal also
decreases. Thus, endpointing becomes more difficult as the
secondary electron emission decreases. Further, as the depth of a
milling operation increases, the number of secondary electrons that
escape the hole becomes less. As such, with less secondary
electrons to detect, high collection efficiency becomes more
important.
One way to improve the collection efficiency of a SED involves the
application of a high voltage (.about.10 kV) to the scintillator
surrounded by a grounded cap to produce a collection electric field
that attracts the secondary electrons. One such system is described
in U.S. Pat. No. 6,630,667 titled "Compact, High Collection
Efficiency Scintillator for Secondary Electron Detection," to Wang
et al. and issued Oct. 7, 2003, which is hereby incorporated by
reference herein. Through the generation of such an electric field,
some of the secondary electrons initially emitted in directions
away from the SED, are attracted to the scintillator thereby
increasing the collection efficiency. Such a system has been
successfully employed in Credence Systems Corporation's IDS
P3X.RTM. FIB system.
However, in FIB systems where it is difficult or impossible to
introduce such an collection field proximate the sample, improving
secondary electron collection efficiency and its attendant image
improvements remains a problem.
Moreover, in some instances, when a high voltage is applied to the
SED, the SED behaves as a focusing lens causing secondary electrons
to strike the scintillator disc within a very small discrete spot.
Over time, a "burn" spot will result with much or all of the
phosphor burned from the scintillator disc, leading to reduced
detection and a reduced lifetime of the disc. The lifetime of the
scintillator disc is further shortened when too many secondary
electrons strike the disc as is the case when the primary ion beam
current is high. In many instances, a less powerful ion beam might
be employed for a particular operation. However, because of the
need to detect secondary electrons, a higher beam current is
employed to cause the emission of a greater number of secondary
electrons.
It is with this background in mind that the inventors developed the
various embodiments of the invention described below.
SUMMARY OF THE INVENTION
The present invention has various aspects. One aspect of the
invention involves a charged particle tool adapted to generate a
charged particle beam and direct the particle beam on a sample,
such as an integrated circuit. The charged particle tool comprises
a platform for supporting the sample and a charged particle
detector arranged proximate the platform. The charged particle
detector may comprise a secondary electron detector employing a
scintillator. A charged particle guide is operably associated with
the charged particle detector. The charged particle guide comprises
a first elongate member and a second elongate member. The elongate
members are arranged to extend from the charged particle detector
toward the platform.
In one particular aspect, the charged particle guide comprises a
first wire and a second wire fabricated with stainless steel,
platinum, nickel, chromium, palladium or other suitable material or
alloy. A first tip region of the first wire may be arranged
adjacent the platform and a second tip region of the second wire
may also be arranged adjacent the platform. Depending on any
particular usage of the invention, the tips may vary in distance
from the platform, may be straight or define one or more angularly
offset sections, and may be coplanar or non coplanar, mirror
images, or each define unique shapes. The first tip region may be
generally parallel with the second tip region. Further, the first
tip region and the second tip region may be arranged to either side
of a location on the platform where the charged particle beam is
incident upon the a sample supported on the platform.
In another particular aspect, the charged particle tool may include
a column housing an ion beam production facility and ion optics,
the column and the platform grounded and arranged in close
proximity. Further, the first and second elongate members of the
charged particle guide may be arranged to introduce a collecting
electric field proximate a sample supported on the platform.
Particularly, the first and the second elongate members are
arranged proximate the platform for collection of secondary charged
particles. To generate an electric field to attract charged
particles, such as secondary electrons, a bias voltage is applied
to the charged particle guide. The shape of the electric field is
dependent upon the length, shape, thickness, and other aspects of
the elongate members. Thus, depending on any particular
implementation of the invention, the elongate members, such as the
wires discussed above, may be of various shapes, sizes, and lengths
to tailor the collecting electric field to the particular use of
the charged particle guide.
The bias voltage applied to the charged particle guide, each
member, the charged particle detector and other components of the
charged particle tool also impact the collecting electric field.
Thus, variations in application of the bias voltage are possible.
In one aspect, a first voltage is applied to the secondary electron
detector housing to define a first collecting electric field and a
second voltage is applied to the charged particle guide to define a
second collecting electric field. The charged particle guide bias
voltage may be less or greater than the charged particle detector
housing bias voltage. To collect negatively charged particles, the
first and the second voltages are positive to create a first
positive collecting electrical field and a second positive
collecting electrical field to attract secondary electrons emitted
from a sample on the platform upon application of the charged
particle beam thereto. Due to the presence of the charged particle
guide, secondary electrons are attracted to the scintillator disc,
but are not concentrated in a discrete location thereon, which
helps to reduce wear.
In the case of a focused ion beam tool or other charged particle
type tool used in IC testing, the first and the second elongate
members are arranged proximate the platform to enhance generation
of an end pointing trace. The first and the second elongate members
may also be arranged proximate the platform to enhance image
generation during a circuit editing procedure.
Another aspect of the invention includes a charged particle
detector assembly. The charged particle detector assembly comprises
a charged particle detector, such as secondary electron detector
employing a scintillator, with a first elongate member and a second
elongate member coupled with the charged particle detector. The
first and second elongate members extend away from the charged
particle detector. Further, at least one electrical connection
point is arranged to supply at least one bias voltage to the first
and the second elongate members. The elongate members may each
comprise a wire fabricated with stainless steel, nickel, chromium,
palladium and platinum.
The secondary electron detector may comprise a housing arranged
circumferentially about the scintillator, and wherein the first
elongate member and the second elongate member are electrically
coupled with the ring. Alternatively, the ring may be coupled with
the elongate members to form an assembly, which is then coupled
with the secondary electron detector housing. In one particular
aspect, the scintillator defines a disk shape defining an axis. The
first elongate member and the second elongate member are coupled
with the ring and each define a first section extending from the
secondary electron detector generally parallel with the axis of the
scintillator. Further, the first and the second elongate members
may each define a second section electrically coupled with the
first section, the second section extending toward the axis of the
scintillator. Still further, the first and second elongate members
may each define a third section electrically coupled with the
second section, the third section extending toward the axis of the
scintillator at less an angle than the second section. Finally, the
first and second elongate members may each define a fourth section
electrically coupled with the third section, the fourth sections
extending substantially parallel the fourth section and further
extending at an angle with respect to the axis of the
scintillator.
It is also possible to include additional elongate members. In one
aspect, at least one additional elongate member is coupled with the
charged particle detector. The first, the second, and the at least
one additional elongate member extending from the charged particle
detector. Moreover, the at least one electrical connection point is
arranged to supply the at least one bias voltage to the first, the
second, and the at least one additional elongate member.
It is also possible to electrically insulate the first elongate
member and the second elongate member from the charged particle
detector housing. A first electrical connection may be adapted to
supply a first bias voltage to the charged particle detector
housing and a second electrical connection may be adapted to supply
a second bias voltage to the first and the second elongate members.
Moreover, the first elongate member may be electrically isolated
from the second elongate member. In such an arrangement, a first
electrical connection provides a first bias voltage to the first
elongate member, a second electrical connection provides a second
bias voltage to the second elongate member and third electrical
connection provides a third bias voltage to the charged particle
detector housing. As discussed above, the shape, size, length and
other features of the elongate members may be varied in any
particular implementation. Additionally, the bias voltages may
differ depending on a particular implementation.
Finally, another aspect of the invention includes a charged
particle guide for directing charged particles toward a charged
particle detector. The charged particle guide comprises a first
conductive elongate member and a second conductive elongate member
coupled with the first conductive elongate member. The first and
the second electrically conductive elongate members are in
electrical communication. Additionally, the first and the second
electrically conductive elongate members are adapted to be operably
associated with a charged particle detector. Upon application of a
bias voltage to the first and the second conductive members an
electrical collection field is generated that is adapted to attract
charged particles to the charged particle detector. The first
conductive member may comprise a first means for attracting charged
particles and the second conductive member may comprise a second
means for attracting charged particles. Additionally, the first and
second attracting means may be coupled via a means for electrically
connecting the first means for attracting charged particles with
the second means for attracting charged particles. An electrical
insulator may be coupled with the first and second conductive
members so that the first and second conductive members are
electrically isolated and biased at a different voltage from the
charged particle guide when connected thereto.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a partial side section view of a focused ion beam tool
employing a charged particle guide, in accordance with one
embodiment of the present invention.
FIG. 2 is an isometric view of a charged particle guide connected
with a ring, in accordance with one embodiment of the present
invention.
FIG. 3 is a top view of the charged particle guide of FIG. 2.
FIG. 4 is a side view of the charged particle guide of FIG. 2.
FIG. 5 is a front view of the charged particle guide of FIG. 2.
FIG. 6 is a partial close up isometric view of the charged particle
guide of FIG. 2 coupled with a secondary electron guide cap and
with the secondary electron guide arranged between the end of the
focused ion beam column and the platform for mounting a device
under test.
FIG. 7A is a contour plot for a constant secondary electron
collection field potential of a scintillator without a charged
particle guide.
FIG. 7B is a plot of the secondary electron trajectory in a charged
particle beam tool without a charged particle guide.
FIG. 8A is a contour plot for a constant secondary electron
collection field potential of a scintillator with a charged
particle guide, the constant potential field penetrating proximate
the device under test.
FIG. 8B is a plot of the secondary electron trajectory in a charged
particle beam tool with a charged particle guide.
FIG. 9A is a secondary electron image of four areas of dielectric
separated by the metal grid pattern therebetween, the secondary
electron image from a focused ion beam tool without a charged
particle guide.
FIG. 9B is a secondary electron image of four areas of dielectric
separated by the metal grid pattern therebetween, the secondary
electron image from a focused ion beam tool with a charged particle
guide.
FIG. 10A is a line scan of the secondary electron image of FIG.
9A.
FIG. 10B is a line scan of the secondary electron image of FIG.
9B.
FIG. 11A is a representative image of the secondary electron
emission characteristics from a high aspect ration hole in an IC
milled by an ion beam, and the secondary electron collection
characteristics of a FIB tool without a charged particle guide.
FIG. 11B is a representative image of the secondary electron
emission characteristics from a high aspect ration hole in an IC
milled by an ion beam, and the secondary electron collection
characteristics of a FIB tool with a charged particle guide, in
accordance with the present invention.
FIG. 12 is an endpoint trace for a focused ion beam hole of 24:1
aspect ratio from a FIB tool employing a charged particle guide
(solid line) and from a FIB tool without a charged particle guide
(dashed line).
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
One aspect of the present invention involves a charged particle
guide that attracts and directs charged particles toward a charged
particle detector, such as a scintillator disk of a secondary
electron detector, and thereby improves its collection efficiency.
The charged particle guide includes a plurality of members
extending from the charged particle detector toward a sample. A
bias voltage, or range of voltages, is applied to the members to
introduce an electric field proximate the sample. When employed in
a tool that generates a charged particle beam and directs the beam
onto the sample, the electric field attracts charged particles
emitted from the sample and directs the charged particles to the
charged particle detector.
One particular configuration of the invention involves a charged
particle guide coupled with a secondary electron detector of a FIB
tool. The charged particle guide, in one particular configuration,
includes a pair of wires extending from secondary electron detector
toward a target IC. The wires are electrically biased and thereby
create an electric field adapted to attract and guide the charged
particles emitted from the IC to the charged particle detector upon
application of the ion beam.
The following discussion focuses primarily on an embodiment of the
invention involving a charged particle guide employed in a FIB
tool. However, a charged particle guide in accordance with the
present invention may be employed to attract and guide charged
articles of various types, in various applications, and in various
charged particle tools. For example, application of a focused ion
beam to a sample may also cause emission of positively charged
secondary ions albeit at a less number than secondary electrons. To
configure a charged particle guide to collect positively charged
secondary ions, a negative bias, at a much higher voltage than for
collection of secondary electrons, is applied to the charged
particle guide. In another example, in an electron beam tool or
"E-Beam" tool, high energy electrons are directed toward a sample,
which also causes the emission of secondary electrons. In an E-Beam
tool employing a charged particle guide conforming to the present
invention, a positive bias is applied to the wires to attract and
guide the secondary electrons.
FIG. 1 is a partial side section view of a FIB tool 100 employing a
charged particle guide 105, in accordance with one implementation
of the present invention. The focused ion beam tool includes a
column 110 that provides a focused ion beam 120 directed toward an
IC 125, which is referred to as a device under test ("DUT") when
being tested, edited, etc. To provide the focused ion beam, the
column includes an ion source, optics, and other structure (not
shown). Further the column may be a coaxial photon-ion column,
which is especially useful for editing flip-chip devices. One
suitable coaxial photon-ion column that may employ a charged
particle guide, in accordance with the present invention, is
described in United States Patent Application Publication US
2003/0102436 titled "Column Simultaneously Focusing a Particle Beam
and Optical Beam," published Jun. 5, 2003, a paper titled "Coaxial
Ion-Photon System" by C. C. Tsao, Q. S Wang, P. Bouchet, and P.
Sudraud published by Elsevier Science Ltd., 2001, and a paper
titled "Editing of IC Interconnects Through Back Side Silicon With
a Novel Coaxial Photon-Ion Beam Column" by Chun-Cheng Tsao, Pierre
Sudraud, Patrick Bouchet and Mark Thomspon, which are hereby
incorporated by reference herein.
A secondary electron detector 130 (the "SED"), is arranged adjacent
the upper end of the column 110 and the DUT 125. The secondary
electron detector comprises a scintillator disk 135, which, as
discussed above, includes a phosphor coating that is adapted to
convert incident electrons into the photons. From the scintillator
disk, the photons travel down a light pipe 142 extending generally
downwardly from the secondary electron detector 130. Although not
shown, the light pipe is coupled with a photomultiplier tube that
converts each incident photon into multiple electrons that may be
used to provide an image of the DUT.
In FIG. 1, a chemistry injector tube 140 (sometimes referred to as
a "gas delivery jet") is arranged to provide gas to the DUT 125
depending on the use of the FIB tool 100. For example, XeF2 may be
used in circuit editing processes to perform edits. Many of the
gases that may be employed in a FIB tool may have an aging effect
on the scintillator disk 135. As will be recognized from the
discussion below, a charged particle guide 105 conforming to the
present invention, allows the secondary electron detector 130 and
scintillator disk to be located further from the DUT and gas
delivery jet, while still maintaining a high collection efficiency.
Arranged further from the gas delivery jet 140, the scintillator
disk 135 is exposed to a lesser amount of chemicals and thus ages
at a lesser rate than with a FIB tool requiring the scintillator be
placed in closer proximity to the gas source. Additionally,
arranging the secondary electron detector 130 further away from the
DUT 125 allows more physical spacing between tool components in the
area around the DUT platform.
The secondary electron detector 130 includes a cap 145 that is
attached to the body of the secondary electron detector and extends
outwardly therefrom. At its outer end, arranged between the
scintillator 135 and the DUT 125, the cap defines a circular
opening 150 with a diameter slightly larger than the diameter of
the scintillator. One suitable secondary electron detector and cap
arrangement is described in U.S. Pat. No. 6,630,667 titled
"Compact, High Collection Efficiency Scintillator for Secondary
Electron Detection" (the "Wang patent") discussed above. In the
Wang patent, the end cap is grounded. However, in some
implementations of the present invention, the end cap 145 is
biased. In one particular implementation, discussed further below,
the end cap is biased to the same voltage as a charged particle
guide 105 conforming to the present invention. In the Wang
arrangement with a grounded cap, secondary electrons may be guided
to a small spot or discrete region on the scintillator disk which
can cause excessive wear of the scintillator material. As discussed
below, besides enhancing the collection efficiency of the secondary
electron detector 130, embodiments of the present invention with a
biased cap and charged particle guide may also cause wider
distribution of the secondary electrons incident on the
scintillator disk 135 reducing wear thereof.
The charged particle guide 105 is electrically coupled with the
detector cap 145 and arranged to extend to a location adjacent the
DUT 125. FIGS. 2 5 illustrate an isometric view, a top view, a side
view, and a front view, respectively, of a charged particle guide
105, in accordance with one implementation of the present
invention. FIG. 6 is a partial close up isometric view of the
charged particle guide of FIG. 2 coupled with a secondary electron
detector cap and with the secondary electron guide arranged between
the end of the focused ion beam column and the platform for
mounting a device under test. In this implementation, the charged
particle guide comprises two elongate members 200. The elongate
members are each electrically coupled with a ring 205, which is
adapted to be fastened around the cap 145. Alternatively, the
elongate members may be electrically connected directly to the
cap.
It is also possible to electrically insulate the detector cap 145
from the charged particle guide 105. In such an arrangement, the
detector cap may be biased at voltage different from the charged
particle guide. Moreover, the detector cap might not be biased, and
a bias only applied to the charged particle guide. Further, it is
also possible to electrically insulate the members of the charged
particle guide and bias each member at a different voltage. For
example, in a charged particle guide 105 employing two wires 200,
each wire might be insulated from the other and biased
independently. Such an arrangement would allow for the generation
of differently shaped collection fields depending on how the wires
were biased as well as other factors, such as the shape and the
length of the wires.
In one implementation, each elongate member 200 is a stainless
steel wire or rod. Stainless steel is used because it is resistant
to the corrosion, degradation, and chemical reactions from the
chemical gases that are often used in various FIB operations. It is
possible that other electrically conductive materials, such as
Nickel (Ni), Chromium (Cr), Platinum (Pt), Palladium (Pd), alloys,
and the like, may be used in place of stainless steel. Although a
wire is shown, it is possible to use other shapes and sized
material depending on the particular environment, tool, and other
factors such as maintaining small cross sections to avoid
intercepting charged particles to be collected.
The wires 200 are mechanically coupled to the ring 205 about 180
degrees apart. The wires may be tungsten inert gas ("Tig") welded,
soldered, spot welded, or otherwise coupled with the ring to
achieve an electrical connection between the ring 205 and the wires
200. As mentioned above, an alternative embodiment of the invention
has the ring electrically isolated from the wires, which allows
different bias voltage application to the ring and wires. The ring
defines a longitudinal axis, which when connected with the cap 145
is arranged coaxially with the longitudinal axis of the
scintillator disk 135.
The wires extend perpendicularly from the ring, and define a
multidimensional shape, including three curves and four generally
straight sections. The first, second, and third sections are in a
first plane, and the fourth section is in a second plane arranged
at an angle with respect to the first plane. The first section 210
is welded to the ring 205, and extends perpendicularly therefrom.
The first curve 215 orients the second section 220 at an angle with
respect to the first section. The second section of both wires 200
are directed generally toward the longitudinal axis of the ring. As
such, the second sections converge. The second radius 225 orients
the 230 third section at an angle with respect to the second
section 220. The third section is also directed toward the
longitudinal axis of the ring, but at a lesser angle than the
second section. As such, the third sections are also converging.
The third radius 235 orients the fourth section 240 in the second
plane. Further, the third radius orients the fourth section of each
wire generally parallel to the other. From FIG. 2, it can be seen
that the two wires 200 form an inverted funnel-like shape, with the
wide end 245 of the funnel at the SED opening 150 and the narrow
end 250 of the funnel directed toward the DUT 125.
The ring 205 includes two tabs 255 extending generally
perpendicular the ring, but in the opposite direction as the wires
200. The tabs define an aperture 260 which may be used to secure
the ring to the cap 145 housing. The aperture may further receive a
bolt for attachment of a wire 155 that provides the bias voltage to
the charged particle guide 105 as well as the cap. In an embodiment
adapted to allow independent bias voltage application to the cap
and the charged particle guide, additional bias wire attachments
are employed. The bias wire may also be directly welded or
otherwise attached to the ring 205. Further, as noted above, it is
possible to connect the charged particle guide 105 wires 200
directly to the cap 145 rather than the ring. The ring, however,
provides a convenient means to retrofit a charged particle guide,
in accordance with the present invention, to secondary electron
detectors 130 of FIB tools already deployed at customer locations
with minimal down time of the FIB tool 100.
In one particular implementation for use in a Credence Systems
Corporation IDS OptiFIB system, a charged particle guide 105
conforming to the invention is fabricated with two stainless steel
wires 200 of 0.34 mm diameter each. The wires should be stiff to
limit or suppress vibrations, which might result in electrical
field noise on the deflection field. Further, stainless steel is an
excellent secondary electron emitter and any electrons striking the
wires would have a greater chance of contributing to the collected
signal. The angle between the primary ion beam 120 and the normal
to the plan spanned by the two wires is about 6o. The separation
between the tips of the two wires is about 5.5 mm. The specific
guide shape near the secondary electron detector cap 145 has less
influence on charged particles guiding and attraction due to the
strong field of the second electron detector 130, in some
implementations. Thus, as the secondary electrons are guided to the
scintillator 135, the influence of the secondary electron detector
field increases dramatically and dominates the influence of the
charged particle guide 105. Measured from the cap to the end of the
guide, the distance is about 31 mm. The end of the guide should not
extend beyond the primary charged particle beam axis, in one
implementation.
The distance from the tips of the wires 200 to the DUT 125 is about
2.85 mm when the DUT is at the optic focal plan of the column 110.
A positive DC voltage of +250 V DC may be applied to a charged
particle guide 105, such as with a Bertan.TM. high voltage power
supply (Model: PMT-30CP-1) which has a specification of less than 6
mV ripple. The bias voltage may be adjusted in any particular
context to obtain an optimum performance. Any suitable low ripple
high voltage supply may be used to supply the bias voltage.
Generally, the wider the separation between the two wires 200, the
higher the bias voltage to obtain optimal secondary electron
attraction and guidance.
The voltage range applied to the charged particle guide 105 should
be balanced with its impact on distorting the placement of the ion
beam 120. As the ion beam has a positive charge, the presence of
the positively charged guide and its field may deflect the beam.
With a low ripple supply, the field of the charged particle guide
is essentially constant. Thus, distortion of the ion beam may be
compensated for to account for a constant deflection. Significant
field strength variations, such as with a noisy voltage supply, may
result in image noise and/or distortion due to variable beam
deflections.
Although some particular charged particle guide arrangements are
shown and discussed herein, other shapes, sizes, and configuration
are possible depending on the particular arrangement of the FIB
tool 100, the distance between the secondary electron detector 130
and the ion beam 120 and/or the DUT 125, the diameter of the
scintillator 135, the shape of the scintillator and any number of
other factors. For example, if the scintillator is farther from the
DUT, then the wires 200 may be of greater length; if the
scintillator disc is larger, then the angles and section lengths
may be greater, if the applied voltage is greater, then the wire
separation may be greater, etc.
Further, rather than a straight wire bent into a configuration with
straight sections separated by radii, it is possible to bend each
wire 200 in a manner that defines a continuous multidimensional
sweeping curve. It is also possible to weld wire/rod sections
together rather than bend a single length of wire.
In alternative embodiments, it is not necessary that the wires 200
be mirror images. Moreover, additional wires, i.e., more than two,
may be employed in other configurations. Further, the axis of the
charged particle guide 105, with various numbers and arrangement of
wires, can be made in parallel, including coaxial, to the axis of
primary charged particle beam. Such a coaxial or parallel
arrangement would be beneficial if the detector used is coaxial to
the column 110. When a bias is applied to the end cap 145 and the
charged particle guide 105, an electric field is created that
attracts secondary electrons from the DUT 125 during a FIB
operation. For attraction of secondary electrons or other
negatively charged particles, the charged particle guide is
positively biased. For attraction of secondary ions or other
positively charged particles, the charged particle guide is
negatively biased. As mentioned above, in one particular
implementation, a bias voltage of +250 V DC is applied to the
charged particle guide. The particular voltage applied in any
particular implementation will vary.
Computer analysis software was first used to verify the concept of
one implementation of a charged particle guide 105, conforming to
the present invention. This software is the EO-3D package which is
capable of three dimensional charged particle ray tracing based on
a finite difference method. FIG. 7A is a contour plot for constant
secondary electron collection field potentials 700 of a secondary
electron detector 130 in the FIB tool 100 of FIG. 1, but without a
charged particle guide. The secondary electron detector is biased
to generate the collection field. In one particular example, the
bias voltage of the secondary electron detector is +9 kV DC. Biased
in such a manner, the secondary electron detector generates the
electric field shown in FIG. 7A. However, the column 110 and the
sample platform holding the DUT 125 are grounded to 0 Volt. As
such, the electric field hardly penetrates into the area between
the column and the DUT.
In a FIB tool 100, if the DUT 125 needs to be arranged closer to
the FIB column 110, and since both DUT and the FIB column are
grounded, a narrow spacing between the column and the DUT results
in less penetration into the DUT region of the collecting electric
field of the secondary electron detector 130. As such, secondary
electrons emitted from DUT will hardly be affected by the
collecting electric field of the secondary electron detector alone.
As a result, many secondary electrons are not collected by the
secondary electron detector. FIG. 7B is a plot of the secondary
electron trajectory 705 in the focused ion beam tool 100 of FIG. 7A
without a charged particle guide 105. As discussed above, the
secondary electrons are emitted in all directions from the DUT 125.
Because the collecting electric field does not penetrate closely to
the DUT, secondary electrons 710 initially directed at an angle
toward the secondary electron detector 130 may be attracted by the
collecting field of the secondary electron detector and strike the
scintillator 135. However, secondary electrons initially directed
at angles away from the secondary electron detector do not come
under the influence of the electric field and are not detected.
In contrast to FIG. 7A, FIG. 8A is a contour plot for constant
secondary electron collection field potentials 800 of a secondary
electron detector 130 employing a charged particle guide 105, in
accordance with one implementation of the present invention. In
this example, a +9 kV DC potential is applied to the secondary
electron detector and a +250 V DC potential is applied to the
charged particle guide and the detector housing. Note, FIGS. 8A and
8B, show the plan along the axis of the charged particle guide, so
one of the charged particle guide wires 200 is shown in hidden line
and the other is not shown. A collecting electric field potential
805 generated by the charged particle guide 105 penetrates into the
area adjacent the DUT 125. In this example, the overall collecting
electric field is a combination of the fields generated by the
voltage applied to the secondary electron detector 130 and the
voltage applied to the charged particle guide. It will be
recognized that the collecting field adjacent the DUT is primarily
a result of the charged particle guide 105 bias voltage whereas the
collecting electric field adjacent the body of the secondary
electron detector 130 and the scintillator disk 135 is primarily a
result of the much higher voltage applied to the secondary electron
detector.
FIG. 8B is a plot of the secondary electron trajectory 810 in a FIB
100 employing a charged particle guide 105 in the arrangement shown
in FIG. 8A. In contrast to FIG. 7B, as shown in FIG. 8B a majority
of the secondary electrons 810 initially come under the influence
of the collecting electric field 800 of the charged particle guide
105 and secondary electron detector 130 and are guided toward the
scintillator disk 135. As such, FIG. 8B illustrates the dramatic
improvement in secondary electron collection efficiency for a
secondary electron detector 130 and FIB tool 100 employing a
charged particle guide 105, in accordance with the present
invention.
By positively biasing two metal wires 200 coupled with the
secondary electron detector 130, a collecting electric field is
introduced into a region where the electric field would otherwise
hardly penetrate. Under the influence of the electric field from
these electrically biased wires, secondary electrons are guided
along the guide towards the secondary electron detector
scintillator 135. As a result, the secondary electron detector
collection efficiency can be greatly enhanced, which advantageous
result is further illustrated in the FIB image brightness and
contrast illustrated at FIGS. 9A and 9B, discussed below.
FIG. 9A is a representative secondary electron image 901 of four
areas of dielectric 900 separated by a metal grid pattern 905
therebetween. The representative image was generated based on an
actual secondary electron image from a FIB tool 100 without a
charged particle guide 105. In contrast, FIG. 9B is a
representative secondary electron image 902 of the four areas of
dielectric 910 separated by the metal grid pattern 915 therebetween
based on an actual secondary electron image from a FIB tool 100
with a charged particle guide 105, in accordance with one
embodiment of the present invention. In a FIB image, bright (white)
areas are associated with a high concentration of secondary
electron detection whereas gray to dark areas are associated with a
lower to nearly no secondary electron detection. Metal generally
emits more secondary electrons than a dielectric. Thus, the image
generated from secondary electrons emitted from a metal region upon
application of an ion beam 120 will be brighter that the image of a
dielectric.
In FIG. 9A, it can be seen that the image 901 of the metal area 905
is gray, whereas the image 902 of the metal area 915 of FIG. 9B
(from a FIB tool 100 with a charged particle guide 105) is much
brighter. The dielectric area 900 of the image of FIG. 9A is
somewhat darker than the dielectric area 910 of FIG. 9B, as would
be expected due to the greater detection of secondary electrons
with the charged particle guide 105. Nonetheless, due to the
dramatically whiter metal area 915 of FIG. 9B caused by enhanced
collection efficiency of the secondary electron detector 130, the
contrast between the metal 915 and the dielectric 910 of FIG. 9B is
greater than the contrast of FIG. 9A. Thus, the boundary between
metal and dielectric is easier to visually identify in FIG. 9B,
which provides the user with an enhanced ability to identify IC
elements for circuit editing, visually verify circuit editing, and
performing other operations using the FIB tool 100 and image more
accurately than possible with some FIB tools not employing a
charged particle guide 105.
FIG. 10A is a line scan graph 1000 of the actual secondary electron
image used to generate the representative image 901 of FIG. 9A, and
FIG. 10B is a line scan graph 1005 of the actual secondary electron
image used to generate the representative image 902 of FIG. 9B. The
horizontal axis 1010 represents pixel locations associated with the
raster scan image generation used to create the image of FIGS. 9A
and 9B. The vertical axis 1015 represents the brightness, which is
proportional to the number of detected secondary electrons, for any
particular pixel location. Each graph illustrates a curve with
three high regions 1020 associated with a first concentration of
detected secondary electrons from the metal regions. Each graph
also illustrates two low regions 1025, between the high regions,
associated with a second concentration of detected secondary
electrons from the dielectric regions.
First, it can be seen that the high regions 1020 in the second
graph 1005 are about twice the height of the high regions in the
first graph 1000. As such, at least twice as many secondary
electrons were detected with a FIB tool 100 employing a charged
particle guide 105 conforming to the present invention (FIG. 10B)
as opposed to a FIB tool not employing a charged particle guide
(FIG. 10A). Second, with respect to the low regions 1025 of
secondary electrons detected from the dielectric, it can be seen
that slightly more, but not nearly twice the number of secondary
electrons were detected with a FIB tool employing a charged
particle guide 105 conforming to the present invention (FIG. 10B)
as opposed to a FIB tool 100 not employing a charged particle guide
(FIG. 10A). As mentioned above, metal generally emits many more
secondary electrons than does a dielectric. Due to the high
collection efficiency of the charged particle guide, more secondary
electrons are collected from both the metal 905 and dielectric 900
regions as reflected in FIG. 1A. However, due to the differences in
emission characteristics, the contrast (the difference in secondary
electrons from metal and dielectric), is greater in FIG. 10B than
in FIG. 10A, and thus provides the sharper image of FIG. 9B.
The enhancement of collection efficiency is not limited to
secondary electrons emitted from the surface of a sample. Secondary
electrons emerging from a small opening of a high aspect ratio hole
will be collected efficiently as well. The mechanism for secondary
electrons to escape from a small hole of high aspect ratio has been
studied and understood. The stronger collecting field introduced by
a charged particle guide 105 conforming to the present invention
helps those secondary electrons that have made their ways to near
the opening of and to escape from the high aspect ratio hole. Under
the guidance of the two wires 200, those secondary electrons are
then traveling towards and collected by the SED 130. Therefore,
very important end-pointing signals are made stronger with this
invention.
FIG. 11A is a diagram illustrating a focused ion beam tool 100
directing an ion beam 120 to mill a high aspect ratio hole 1100 in
an integrated circuit 1105. The integrated circuit includes a
number of metal layers 1110 between the dielectric 1115 and the ion
beam is incident upon the target metal layer 1120, which is the
"end point" 1125 of the milling operation. It is the objective of
the FIB operator, to mill the hole between the upper metal layers
and stop the milling operation when the target metal layer is
reached. It can be seen that at the depth of the target metal layer
1120, many of the secondary electrons are incident upon the side
walls 1130 of the hole 1100 and do not escape. In FIG. 11A, the
secondary electron detector 130 does not include a charged particle
guide 105 to attract secondary electrons escaping from the hole
1100. Thus, the secondary electrons escaping the hole do not come
under the attraction of the secondary electron detector 130 because
the collecting electron field cannot adequately penetrate into the
area adjacent the hole.
In contrast, FIG. 11B is a diagram illustrating a FIB tool 100
employing a charged particle guide 105 conforming to the present
invention. In this example, the two wires 200 of the charged
particle guide are disposed in only one plane. The FIB tool is
directing an ion beam 120 to mill a high aspect ratio hole 1100 in
the integrated circuit 1105. As with FIG. 11A, the IC includes a
number of metal layers 1110 between the dielectric 1115, and the
ion beam is incident upon the target metal layer 1120, which is the
"end point" 1125 of the milling operation. As with FIG. 11A, many
of the secondary electrons emitted from the target metal layer are
incident upon the side walls 1130 of the hole and thus do not
escape. In contrast to FIG. 11A, the secondary electrons 1135 that
do escape the hole 1100 come under the influence of the collecting
electric field generated by the charged particle guide 105 and are
thus directed to and detected by the secondary electron detector
130.
FIG. 12 is an end point trace illustrating the number of secondary
electrons detected through the depth of a 0.25.times.0.25 .mu.m
24:1 aspect ratio hole, similar to that as illustrated in FIGS. 11A
and 11B. The solid line is reflective of the secondary electrons
detected by a FIB tool 100 employing a charged particle guide 105
conforming to the present invention. The dashed line is reflective
of the secondary electrons detected by a FIB tool 100 without a
charged particle guide 105. It can be seen that when the milling
operation is first begun, many secondary electrons are initially
detected from the surface of the IC 1105. With the charged particle
guide 105, more secondary electrons are detected. As the milling
operation progresses and the hole deepens into the dielectric 1115,
less secondary electrons are emitted and detected by both FIB
configurations until the curve flattens out and little or no
secondary electrons are detected. However, when the ion beam 120
reaches the target metal layer 1120, such as metal 1, more
secondary electrons are emitted. At this point, there is a small
rise 1200 in the curve from the FIB tool 100 employing the charged
particle guide 105. Thus, an increase in secondary electrons is
detected. Based on this end point indicator 1200, the operator
knows that he has reached the target layer 1120 and may end the
milling operation. Due to the low collection efficiency of the
secondary electron detector 130 without a charged particle guide,
little or none of the secondary electrons emitted at the target
metal are detected and there is no visible rise in the curve for
the FIB tool. As such, the operator is not aware that he has
reached the end point of the milling operation.
Since aspects of this invention are based on conservation of
angular momentum, a single wire can also enhance the collection
efficiency. But investigation indicates that a two-wire guide is
more efficient than a single wire perhaps because less electrons
are intercepted by the wires. Compared with using a single wire, a
two-wire secondary electron guide 105 also causes less distortion
to the primary ion beam.
As mentioned above, unlike some existing FIB tools, the secondary
electron detector cap 145 is no longer grounded, but biased at the
same voltage as the charged particle guide 105. As a result,
secondary electrons are no longer focused into one small spot at
the scintillator disc 135 and the lifetime of the scintillator disc
can, therefore, be prolonged. The lifetime of the scintillator disc
can further be improved by reducing the bias voltage on the
secondary electron guide so that less secondary electrons will
strike the scintillator disc when the primary beam current is high.
This is possible because the charged particle guide 105 initially
attracts the secondary electrons. Further, in some instances, due
to the enhanced collection efficiency of the FIB tool 100, lower
current beams may be employed in some applications such as high
aspect ratio hole drilling.
Although various representative embodiments of this invention have
been described above with a certain degree of particularity, those
skilled in the art could make numerous alterations to the disclosed
embodiments without departing from the spirit or scope of the
inventive subject matter set forth in the specification and claims.
All directional references (e.g., upper, lower, upward, downward,
left, right, leftward, rightward, top, bottom, above, below,
vertical, horizontal, clockwise, and counterclockwise) are only
used for identification purposes to aid the reader's understanding
of the embodiments of the present invention, and do not create
limitations, particularly as to the position, orientation, or use
of the invention unless specifically set forth in the claims.
Joinder references (e.g., attached, coupled, connected, and the
like) are to be construed broadly and may include intermediate
members between a connection of elements and relative movement
between elements. As such, joinder references do not necessarily
infer that two elements are directly connected and in fixed
relation to each other.
In some instances, components are described with reference to
"ends" having a particular characteristic and/or being connected to
another part. However, those skilled in the art will recognize that
the present invention is not limited to components which terminate
immediately beyond their points of connection with other parts.
Thus, the term "end" should be interpreted broadly, in a manner
that includes areas adjacent, rearward, forward of, or otherwise
near the terminus of a particular element, link, component, member
or the like. In methodologies directly or indirectly set forth
herein, various steps and operations are described in one possible
order of operation, but those skilled in the art will recognize
that steps and operations may be rearranged, replaced, or
eliminated without necessarily departing from the spirit and scope
of the present invention. It is intended that all matter contained
in the above description or shown in the accompanying drawings
shall be interpreted as illustrative only and not limiting. Changes
in detail or structure may be made without departing from the
spirit of the invention as defined in the appended claims.
* * * * *